Subject: Advanced Network Technologies

Topic: WAN Technologies

Difficulty: Low

• ATM standard defines two types of ATM connections, virtual path connections (VPCs), which contain virtual channel connections (VCCs).
• A virtual channel connection (or virtual circuit) is the basic unit, which carries a single stream of cells, in order, from user to user.
• A collection of virtual circuits can be bundled together into a virtual path connection.
• A virtual path connection can be created from end-to-end across an ATM network. In this case, the ATM network does not route cells belonging to a particular virtual circuit.
• All cells belonging to a particular virtual path are routed the same way through the ATM network, thus resulting in faster recovery in case of major failures.
• In this case, all the switches within the ATM network are only VP switches, i.e. they switch the cells only on the basis of VPIs.
• Only the switches, which are connected to the subscribers are VP/VC switches, i.e. they use both VPIs and VCIs to switch the cell.
• This configuration is usually followed so that the intermediate switches can do switching much faster.
• An ATM network also uses virtual paths internally for the purpose of bundling virtual circuits together between switches.
• Two ATM switches may have many different virtual channel connections between them, belonging to different users.
• These can be bundled by two ATM switches into a virtual path connection.
• This can serve the purpose of a virtual trunk between the two switches.
• This virtual trunk can then be handled as a single entity by perhaps, multiple intermediate virtual paths cross connects between the two virtual circuit switches.